Research

Nanoporous graphene and other 2D materials for Gas Separation

Graphene, a
one-atom-thick planar allotrope of carbon, has extraordinary thermal
and electrical conductivity and mechanical strength. Moreover,
experiment and
theory indicate that a single graphene sheet is impermeable to gases
even as small as helium; pores are required for transmission of atoms
or molecules. Interestingly, these types of pores can be synthesized
in a bottom-up fashion using the tools of organic chemistry. We have
been investigating how these nanoporous forms of graphene can be used
for chemical and isotopic separations. Because quantum tunneling
plays a role in the transmission of atoms through these pores, even at
room temperature, this leads to new types of effects which have not
previously been utilized in separations. Alternatively, fluorinating
these structures can lead to selective adsorption of molecules on the
surface, which can be used for gas separations related to pollution control and renewable energy.

Organic semiconductors

High-performance organic semiconductors are important for the development of low-cost printable electronics and large-area (e.g., photovoltaic) applications. However, compared to traditional inorganic semiconductors, organic semiconductors typically have low charge mobility, a measure of how rapidly and easily charges can move through the material. Charge mobility determines both the switching speed (important for printable electronics and radio-frequency identification (RFID) tag applications), and is also related to efficiency losses in organic photovoltaics. In collaboration with the groups of Prof. Zhenan Bao (Stanford), Prof. Alan Aspuru-Guzik (Harvard), and Prof. Sergio Granados-Focil (Clark) we have recently demonstrated how theoretical calculations can guide the development of new high-performance organic semiconductor materials. Current work in this area includes development of new n-channel organic semiconductors with high performance and developing computationally efficient schemes for identifying the best candidate molecules.

One
spin-off of this work involves polyaromatic hydrocarbon (PAH)
(“nanographene” or “graphene nanoparticles”) molecules. Some of the
earliest organic semiconductors, such as tetracene and pentacene,
belong to this class of molecules. New synthetic methods make it
possible to construct larger, more stable PAHs with a variety of
shapes, providing new ways to tune the material properties. Based on
theoretical calculations we have been able to show that a large class
of graphene nanoparticles exhibit efficient multiple exciton
generation, a process where the excess energy contained in high-energy
photons is captured and converted into an additional charge excitation
in the material rather than being dissipated as heat. MEG can be used
to create solar cells that exceed the Shockley-Queisser thermodynamic
efficiency limit, and thus have the potential to improve the
performance and reduce the cost of solar cells.

Organically-templated Inorganic materials

This project centers on
non-centrosymmetric
organically-templated inorganic solids, in collaboration with
experimental work by my Haverford colleague Prof. Alexander
Norquist. Non-centrosymmetry (i.e., absence of an inversion
symmetry) of these crystals gives them important technological
properties, such as piezoelectric,
pyroelectric,
non-linear
optical, and
ferroelectric
effects. Since only a small fraction of crystals are
non-centrosymmetric, it is important to develop new ways to make these
types of materials, and understanding the factors that lead to
non-centrosymmetry is necessary so we can rationally design new
materials. By combining experiment and computation, we have been
working to quantify the “charge density matching” model for
understanding the molecular factors giving rise to different layer
morphologies and crystal symmetries of these types of materials, and to apply new ways of assigning local atomic charges from
planewavepseudopotentialdensity functional calculations using the iterative-Hirshfeld method. More recently, we have been exploring the use of the Non-Covalent Interaction (NCI) index, and applications of data-mining to accelerate the discovery of new solid-state materials.